This invention relates to electrodes for rechargeable electrochemical cells and more particularly to a new and improved negative electrode and a new and improved process and apparatus for making a negative electrode for a rechargeable electrochemical cell.
A negative electrode for a nickel-cadmium electrochemical cell may be made by a number of alternative processes known in the art. For example, the electrode may be made by a so-called sintered electrode process wherein a porous nickel carrier body is first sintered to a metallic current conducting substrate and then chemically impregnated with the active cadmium material involved in the electrochemical reactions of the cell during cell charge and discharge. Another alternative process for making a negative electrode for a rechargeable nickel-cadmium cell is commonly referred to as a pasted electrode process wherein the chemically active cadmium material is mixed with water and a binder to form a slurry or paste. The paste is spread directly onto the metallic current conducting metallic substrate and then dried.
Yet another alternative process electro-deposits cadmium directly onto a continuously moving strip of current conducting metallic substrate. The strip of material, with the cadmium deposited thereon, is then cut to size to form individual electrodes which are subsequently assembled into cells. One method of electro-depositing cadmium on a continuous metallic strip is described in U.S. Pat. Nos. 4,169,780 and 4,180,441. Briefly stated, these patents describe a process and machine wherein a negatively polarized continuous strip of metal is passed through a cadmium sulfate plating bath containing a positively polarized cadmium electrode. Current flow between the negatively polarized strip and the positively polarized cadmium electrode and through the plating bath is effective to disassociate cadmium ions from the cadmium anode and deposit cadmium metal on the negatively polarized continuous strip comprising the cathode.
The aforementioned sintered and pasted processes, which differ substantially from each other, have met with some success in producing electrodes found adequate for commercial nickel-cadmium cells. However, methods and apparatus, currently known in the art, for electro-depositing cadmium directly upon a continuous metallic strip have not proven to be entirely satisfactory in producing cadmium negative electrodes fully suitable for use in rechargeable nickel-cadmium cells. By way of example, one problem associated with electrodeposited cadmium negative electrodes made by the methods and apparatus known in the art arises from the fact that the continuous strip, and thus the electrodes, exit the electrodeposition machine and process in a fully charged state; that is, the active material on the negative electrode is in the form of cadmium metal. Yet, a rechargeable electrochemical cell when fully assembled and fully closed must have the state of charge of its positive electrode carefully matched in a predetermined relationship to the state of charge of its negative electrode. Matching of the states of charge is necessary in order for the closed cell to operate properly during subsequent charge and discharge cycles. Without careful matching of the state of charge of the positive electrode to the state of charge of the negative electrode, significant deficiencies in the subsequent performance of the cell may result. For example, initial charging of a closed cell having a fully charged negative electrode and a substantially uncharged positive electrode will cause electrolysis of the water comprising part of the electrolyte solution and the evolution of oxygen and hydrogen gases. As these gases are evolved, the internal pressure of the cell increases. When the pressure rises sufficiently to exceed the opening pressure of the cell's safety vent mechanism the safety vent will open and expel the gases and electrolyte droplets from the cell container. Thus, during initial charging of a closed cell, the quantity of electrolyte within the cell is reduced and the electrolyte remaining in the cell may be insufficient to enable the cell to achieve optimum operation and performance during subsequent charge and discharge cycles
For this reason, then, when a fully charged negative cadmium electrode made by electrodeposition is assembled with a positive electrode at less than full charge, techniques known in the art provide for initially charging the assembled cell prior to final cell closure. That is, the cell is initially charged under conditions permitting the evolved gases to escape from the cell container. After initial charging has caused the positive electrode to reach a fully charged state, charging of the cell is terminated and then water is added to the cell to replenish that lost during the initial charge episode. This method of attaining a finally closed nickel-cadmium rechargeable cell is both time consuming and costly. The method introduces a separate, discrete manufacturing step after the cell has been assembled from its individual components. It consumes electrolyte during initial charging. Furthermore, because the amount of water lost during the initial charging episode may vary from cell to cell, the amount of water required for replenishment purposes will vary from cell to cell. Adding the same amount of water to each cell produced in high volume manufacture may result in performance deficiencies in at least a portion of the cell produced. Furthermore, it would be impractical to measure the amount of water lost by each cell in a production line for purposes of determining the amount of water required for replenishment of each cell. For all of these reasons, this technique for providing a finally closed cell is not entirely satisfactory.
Prior art techniques for electro-depositing cadmium on a continuous metallic strip for use in the manufacture of electrodes have been less than satisfactory for other reasons. It has been observed that nickel-cadmium cells assembled with an electro-deposited negative electrode have exhibited a condition commonly referred to as fade; that is, the capacity of the cell to deliver energy during discharge decreases with the number of repetitive charge/discharge cycles. This condition has been attributed to the tendency of cadmium hydroxide, which is the form in which the cadmium exists in the negative electrode when the cell is discharged, to dissolve to a slight degree in the liquid electrolyte within the cell. The cadmium hydroxide, while in the dissolved state in very low concentrations, may migrate from its initial location in the electrode. Eventually, for instance when the cell is charged, the cadmium may precipitate out of solution. If the dissolved cadmium hydroxide is in the separator material when precipitation occurs, the precipitated cadmium will reside in the separator. Over a number of repeated charge and discharge cycles, more and more cadmium is dislocated away from the electrode and into the separator. Dislocation of the cadmium in this manner diminishes the deliverable energy capacity of the electrode. Furthermore, dislocation of the cadmium from the electrode to the separator begins to form a conductive path through the separator from the negative electrode to the positive electrode. When the conductive path through the separator becomes well established, the positive electrode will be shorted to the negative electrode and the useful life of the cell will be at an end.
If, on the other hand, the dissolved cadmium is near another cadmium particle when precipitation occurs, precipitation may result in agglomeration; that is, a growth in the size of the cadmium particle which decreases the ratio of surface area of the cadmium particle to its volume. A decrease in this ratio reduces the accessibility of the electrolyte to the active cadmium material resulting in a reduction in the capacity of the cell to deliver energy.
With electrodeposition methods and machines known in the art, cadmium is deposited in crystalline form on a continuous strip of metal substrate. The cadmium coated strip is then cut into individual electrodes for assembly into a rechargeable electrochemical cell. The capacity for the electrochemical cell to deliver energy is very dependent upon the accessibility of the deposited cadmium to the electrolyte within the cell; that is to say, dependent upon the degree to which the deposited cadmium is utilized in the electrochemical reactions attendant the cell's charge and discharge cycles. This utilization capacity is expressed in terms of the number or fraction of milliampere hours of energy delivered per gram of cadmium hydroxide in the form of cadmium. The utilization capacity of an electro-deposited cadmium electrode is largely determined by the amount of cadmium in the electrode and the structure or form in which the cadmium is electro-deposited on the metallic electrode substrate. It is believed that deposition of the cadmium in the form of large crystals does not provide cadmium in a form sufficiently accessible to the electrolyte to effect optimum utilization. it is further believed that less than optimum utilization capacity results from the inability of the electrolyte to contact the cadmium deep within the crystalline structure. Prior art methods and machines have not been entirely successful in achieving optimum utilization capacity at least partly because of the inability to inhibit large cadmium crystal growth.
Good adherence between the electro-deposited cadmium and the continuous metallic substrate strip is an important consideration in the manufacture of an electrode for an electrochemical cell. Achieving good adherence during electrodeposition permits the electrode to be readily compatible with subsequent cell assembly operations without loss of cadmium particles for the substrate surface. In order for the electro-deposited cadmium electrode to be able to sustain high speed assembly and winding into a wound electrode assembly configuration, the bond between the cadmium particles and the substrate must be sufficiently strong to preclude the cadmium deposit from flaking from the surface of the substrate. Similarly, the electrode must be capable of withstanding the rigors of insertion into a cell container without peeling from the surface of the substrate. However, the surface of the continuous nickel-plated steel strip substrate often becomes contaminated with various substances such as oils, greases and surface oxides prior to electrodeposition of cadmium. These contaminants can adversely affect the bond between the continuous strip and the electro-deposited cadmium particles resulting in an electrode ill-equipped to sustain the rigorous handling associated with subsequent cell assembly and manufacturing operations.
As known in the art, processes and machines for electro-depositing cadmium on a continuous strip for use as electrodes in rechargeable electrochemical cells have been notoriously slow. That is to say the speed at which the strip moves through the electrodeposition plating bath and out of the electrodeposition machine is very slow. Among other factors, the amount of cadmium deposited on the strip is a function of the amount of time that the strip is in the electrodeposition plating bath and the current density at the interface between the strip's surface and the plating bath. This surface current density is defined as the current applied to the strip divided by the area of the strip exposed to the plating bath. To deposit the same amount of cadmium at a faster strip speed as would be deposited at a slower speed, the length of the electro-deposition tank must be increased while holding surface current density constant For example, holding surface current density constant and doubling the length of the deposition tank will permit the speed of the strip to be doubled, thus doubling the output of the machine, while still depositing the same amount of cadmium on the strip.
However, in prior art methods and machines, difficulties have arisen in maintaining the surface current density (as heretofore defined) constant as the length of the electrodeposition tank is increased beyond a certain limit. For all practical purposes, these difficulties limit the speed at which the strip may be passed through the electro-deposition tank without reductions in the amount of deposited cadmium. More specifically, maintaining current density constant as the length of the electro-deposition tank is increased is accomplished by increasing the current applied to the strip. Prior art machines have applied the current to the strip at a point just prior to entrance of the strip into the deposition tank. Thus, increases in current required to keep surface current density constant will increase the density of the current through the cross-section of the strip. For example, if the length of the tank is doubled, maintaining a constant surface current density will require a doubling of the cross-sectional current density. Doubling of the cross-sectional current density increases the heat generated in the strip. The additional amount of heat generated is the limiting factor of attaining advancement in the speed at which prior art processes and machines operate.
Accordingly, it is therefore an object of the present invention to provide an electrode suitable for a rechargeable electrochemical cell and a process and apparatus for making such an electrode.
It is another object of the present invention to provide an electrode for a rechargeable electrochemical cell by the electrodeposition of active material on a continuously moving metallic strip.
It is yet another object of the present invention to provide an electrode for a rechargeable electrochemical cell which, when assembled into a rechargeable electrochemical cell, is at a state of charge enabling the cell to be initially charged without causing the evolution of gases in quantities causing excessive pressure.
It is still another object of the present invention to provide an electrode for a rechargeable electrochemical cell by the deposition of active material on a continuously moving strip and to provide an electrode which, when assembled into the cell, is at a state of charge enabling the cell to be initially charged after the cell container has been sealed or closed.
It is yet another object of the present invention to provide an electrode for a rechargeable electrochemical cell by electro-depositing active material on a continuous moving strip wherein the electrode is resistant to migration and agglomeration of the active material during repetitive charging and discharging of the cell.
It is still another object of the present invention to provide an electrode for a rechargeable electrochemical cell by depositing active material on a continuously moving strip in a crystal size and configuration achieving improved electrochemical utilization capacity.
It is yet another object of the present invention to provide an electrode for a rechargeable electrochemical cell by depositing active material on a continuously moving strip in a manner achieving good adherence of the active material to the strip.
It is still another object of the present invention to provide a process and apparatus for achieving all of the aforementioned objectives.
It is yet another object of the present invention to provide a process and apparatus for manufacturing an electrode for a rechargeable electrochemical cell by electro-depositing active material upon a continuously moving metallic strip at speeds exceeding those heretofore known in the art.